The proposed research is concerned with the function of autogenetic reflex pathways in the control of normal movement and posture. The experiments have been designed to test the stiffness hypothesis, which states that the combined action of feedback from muscle spindles and tendon organs is to regulate muscular stiffness, so that the limbs will react to mechanical disturbances in a predictable fashion. The hypothesis has been shown to apply in the decerebrate animal, in which force feedback is actively inhibited and regulated stiffness is abnormally high. Attempts will be made to lower the regulated stiffness of the soleus muscle in the premammillary cat using microstimulation of areas in the red nucleus and other loci known to influence transmission in 1a and 1b pathways. The stiffness hypothesis will be tested under these conditions which would presumably be a better model of normal postural regulation than the conventional decerebrate preparation. Using this procedure, 1) regulation of stiffness will be evaluated by checking the linearity of the response properties of the muscle. Control experiments will be performed to confirm that force feedback has been strengthened. 2) Effects will be classified as causing changes in stiffness or causing changes in the level of activation without altering stiffness. The possibility that stiffness can be continuously graded will be checked. 3) The contributions of rate coding and recruitment to changes in activation during stimulation of the brainstem will be assessed, using electromyographic recordings of single motor units under static and dynamic conditions.